Relax mc validations in the CLI to avoid errors

Followup to #13061

When the CLI queried for Links in the cluster, we were assuming the CLI version matched the linkerd version in the cluster, and so we validated that all fields should be present.

This change relaxes that assumption by setting defaults for the new fields `failureThreshold` and `timeout`, so we don't get an error for example when trying to run `linkerd multicluster uninstall` on a cluster with an older `Link` CRD.

pkg/multicluster/link.go

@@ -50,6 +50,10 @@ type (
 		Selector                      metav1.LabelSelector
 		RemoteDiscoverySelector       metav1.LabelSelector
 	}
+
+	ErrFieldMissing struct {
+		Field string
+	}
 )
 
 // LinkGVR is the Group Version and Resource of the Link custom resource.
@@ -63,6 +67,10 @@ func (ps ProbeSpec) String() string {
 	return fmt.Sprintf("ProbeSpec: {path: %s, port: %d, period: %s}", ps.Path, ps.Port, ps.Period)
 }
 
+func (e *ErrFieldMissing) Error() string {
+	return fmt.Sprintf("Field '%s' is missing", e.Field)
+}
+
 // NewLink parses an unstructured link.multicluster.linkerd.io resource and
 // converts it to a structured internal representation.
 func NewLink(u unstructured.Unstructured) (Link, error) {
@@ -326,7 +334,13 @@ func newProbeSpec(obj map[string]interface{}) (ProbeSpec, error) {
 
 	failureThresholdStr, err := stringField(obj, "failureThreshold")
 	if err != nil {
-		return ProbeSpec{}, err
+		var efm *ErrFieldMissing
+		if errors.As(err, &efm) {
+			// older Links might not have this field
+			failureThresholdStr = fmt.Sprint(DefaultFailureThreshold)
+		} else {
+			return ProbeSpec{}, err
+		}
 	}
 	failureThreshold, err := strconv.ParseUint(failureThresholdStr, 10, 32)
 	if err != nil {
@@ -335,7 +349,13 @@ func newProbeSpec(obj map[string]interface{}) (ProbeSpec, error) {
 
 	timeoutStr, err := stringField(obj, "timeout")
 	if err != nil {
-		return ProbeSpec{}, err
+		var efm *ErrFieldMissing
+		if errors.As(err, &efm) {
+			// older Links might not have this field
+			timeoutStr = DefaultProbeTimeout
+		} else {
+			return ProbeSpec{}, err
+		}
 	}
 	timeout, err := time.ParseDuration(timeoutStr)
 	if err != nil {
@@ -368,7 +388,7 @@ func newProbeSpec(obj map[string]interface{}) (ProbeSpec, error) {
 func stringField(obj map[string]interface{}, key string) (string, error) {
 	value, ok := obj[key]
 	if !ok {
-		return "", fmt.Errorf("Field '%s' is missing", key)
+		return "", &ErrFieldMissing{Field: key}
 	}
 	str, ok := value.(string)
 	if !ok {